Hypoplastic left heart syndrome is the most common lethal cardiac malformation of the newborn. Its treatment , apart from heart transplantation , is the Norwood operation. The initial procedure for this staged repair consists of reconstructing a circulation where a single outlet from the heart provides systemic perfusion and an interpositioning shunt contributes blood flow to the lungs. To better understand this unique physiology, a computational model of the Norwood circulation was constructed on the basis of compartmental analysis. Influences of shunt diameter, systemic and pulmonary vascular resistance, and heart rate on the cardiovascular dynamics and oxygenation were studied. Simulations showed that 1) larger shunts diverted an increased proportion of cardiac output to the lungs, away from systemic perfusion, resulting in poorer O₂ delivery, 2) systemic vascular resistance exerted more effect on hemodynamics than pulmonary vascular resistance, 3) systemic arterial oxygenation was minimally influenced by heart rate changes, 4) there was a better correlation between venous O₂ saturation and O₂ delivery than between arterial O₂ saturation and O₂delivery, and 5) a pulmonary-to-systemic blood flow ratio of 1 resulted in optimal O₂ delivery in all physiological states and shunt sizes.

Color Doppler sonography was used to study umbilical and ductus venosus (DV) flow in 137 normal fetuses between 20 and 38 wk of gestation. Hepatic flows were also evaluated. In all parts of the venous circulation examined, blood flow increased significantly with advancing gestational age. The weight-specific amniotic umbilical flow did not change significantly during gestation (120 ± 44 ml · min⁻¹· kg⁻¹), whereas DV flow decreased significantly (from 60 to 17 ml · min⁻¹· kg⁻¹). The percentage of umbilical blood flow shunted through the DV decreased significantly (from 40% to 15%); consequently, the percentage of flow to the liver increased. The right lobe flow changed from 20 to 45%, whereas the left lobe flow was approximately constant (40%). These changes are related to different patterns of growth of the umbilical veins and DV diameters. The present data support the hypothesis that the DV plays a less important role in shunting well-oxygenated blood to the brain and myocardium in late normal pregnancy than in early gestation, which leads to increased fetal liver perfusion.